The present invention is related to the field of velocity and translation measurement and more particularly to methods and apparatus for the non-contact optical measurement translation and velocity.
Various optical methods for the measurement of the relative velocity and/or motion of an object with respect to a measurement system exist. Each method and apparatus is characterized by the kinds of objects and the kinds of motions on which it operates.
The kind of measurable objects may be broadly divided into several groups, including:
A specially patterned object, for example, a scale.
A reflecting surface, for example, a mirror.
A small particle (or few particles), for example precursor particles or bubbles suspended in fluid.
An optically contrasting surface, for example, a line or dot pattern.
An optically diffuse object, for example, blank paper.
The kind of measurable motions may be broadly divided into several groups, including:
Axial movement toward or away from the measuring device.
Transverse (or tangential) motion, where the spacing between the measuring device and the object is essentially constant.
Rotational motion, where the object orientation with respect to the measurement device is changing.
It is also useful to classify the measurement devices according to the number of simultaneously obtainable measurement directions (one, two or three dimensional) and the number of critical components (light sources, light detectors, lenses, etc.).
It should be noted that a specific method may be related to more than one group in the above classification schemes.
A number of systems capable of non-contact measurement of the transverse velocity and/or motion of objects using optical means have been reported. These methods can include Speckle Velocimetry methods and Laser Doppler Velocimetry methods. Other methods of interest for understanding the present invention are Image Velocimetry methods, homodyne/heterodyne Doppler Velocimetry or Interferometry methods and Optical Coherence Tomography (OCT).
Speckle Velocimetry methods are generally based on the following operational principles:
A coherent light source illuminates the object the motion of which needs to be measured.
The illuminated object (generally an opaque surface) consists of multiple scattering elements, each with its own reflection coefficient and phase shift relative to the other scattering elements.
The individual reflection coefficients and phase shifts are substantially random. At a particular point in space, the electric field amplitude of the reflection from the object is the vector sum of the reflections from the illuminated scattering elements, with an additional phase component that depends on the distance between the point and each element.
The light intensity at a point will be high when contributions generally add in phase and low when they generally add out of phase (i.e., subtract).
On a planar surface (as opposed to a point), an image of random bright and dark areas is formed since the relative phase retardation of the source points depends on the location in the plane. This image is called a xe2x80x9cspeckle image,xe2x80x9d composed of bright and dark spots (distinct xe2x80x9cspecklesxe2x80x9d).
The typical xe2x80x9cspecklexe2x80x9d size (the typical average or mean distance for a significant change in intensity) depends primarily on the light wavelength, on the distance between the object and the speckle image plane and on the size of the illuminated area.
If the object moves relative to the plane in which the speckle image is observed, the speckle image will move as well, at essentially the same transverse velocity. (The speckle image will also change since some scatterers leave the illuminated area and some enter it).
The speckle image is passed through a structure comprising a series of alternating clear and opaque or reflecting lines such that the speckle image is modulated. This structure is generally a pure transmission grating, and, ideally is placed close to the detector for maximum contrast.
The detector translates the intensity of the light that passes through the structure to an electrical signal which is a function of the intensity (commonly a linear function).
When the object moves with respect to the measuring device, the speckle image is modulated by the structure such that the intensity of light that reach the detector is periodic. The period is proportional to the line spacing of the structure and inversely proportional to the relative velocity.
By proper signal analysis, the oscillation frequency can be found, indicating the relative velocity between the object and the measurement device.
For these methods high accuracy frequency determination requires a large detector while high contrast in the signal requires a small detector. A paper by Popov and Veselov, entitled xe2x80x9cTangential Velocity Measurements of Diffuse Objects by Using Modulated Dynamic Specklexe2x80x9d (SPIE 0-8194-2264-9/96), gives a mathematical analysis of the accuracy of speckle velocimetry.
U.S. Pat. No. 3,432,237 to Flower, el. al. describes a speckle velocimetry measuring system in which either a transmission pattern or a pin hole is used to modulate the speckle image. When the pin-hole is used, the signal represents the passage of individual speckles across the pin hole.
U.S. Pat. No. 3,737,233 to Blau et. al. utilizes two detectors in an attempt to solve the problem of directional ambiguity which exists in many speckle velocimetric measurements. It describes a system having two detectors each with an associated transmission grating. One of the gratings is stationary with respect to its detector and the other moves with respect to its detector. Based on a comparison of the signals generated by the two detectors, the sign and magnitude of the velocity may be determined.
U.S. Pat. No. 3,856,403 to Maughmer, et al. also attempts to avoid the directional ambiguity by providing a moving grating. It provides a bias for the velocity measurement by moving the grating at a velocity higher then the maximum expected relative velocity between the surface and the velocimeter. The frequency shift reduces the effect of changes in the total light intensity (DC and low-frequency component), thus increasing the measurement dynamic range and accuracy.
PCT publication WO 86/06845 to Gardner, et al. describes a system designed to reduce the amplitude of DC and low frequency signal components of the detector signal by subtracting a reference sample of the light from the source from the speckle detector signal. The reference signal is proportional to the total light intensity on the detector, reducing or eliminating the influence of the total intensity variations on the measurement.
This reference signal is described as being generated by using a beam-splitter between the measured surface and the primary detector by using the grating that is used for the speckle detection also as a beam-splitter (using the transmitted light for the primary detector and the reflected light for the reference detector) or by using a second set of detectors to provide the reference signal. In one embodiment described in the publication the two signals have the same DC component and opposite AC components such that the difference signal not only substantially removes the DC (and near DC) components but also substantially increases the AC component.
In U.S. Pat. No. 4,794,384, Jackson describes a system in which a speckle pattern reflected from the measured surface is formed on a 2 D CCD array. The surface translation in 2 dimensions is found using electronic correlation between successive images. He also describes an application of his device for use as a xe2x80x9cpadless optical mouse.xe2x80x9d
Image velocimetry methods measure the velocity of an image across the image plane. The image must include contrasting elements. A line pattern (much like a grating) space-modulate the image, and a light-sensitive detector is measuring the intensity of light that pass through the pattern. Thus, a velocity-to-frequency relation is formed between the image velocity and the detector AC component. Usually, the line pattern moves with respect to the detector so that the frequency is biased. Thus, the direction ambiguity is solved and the dynamic range expanded.
A paper by Li and Aruga, entitled xe2x80x9cVelocity Sensing by Illumination with a Laser-Beam Patternxe2x80x9d (Applied Optics, 32 p.2320, 1993) describes image velocimetry where the object itself is illuminated by a periodic line structure (instead of passing its image through such a pattern). The line pattern is obtained by passing an expanded laser beam through periodic transmission grating (or line structure). According to the suggested method the object still needs to have contrasting features.
There exist a number of differences between Image Velocimetry (IV) and Speckle Velocimetry (SV). In particular, in SV the random image is forced by the coherent light source, whereas in IV an image with proper contrasting elements is already assumed. Furthermore, in SV the tangential velocity of the object is measured, whereas in IV the angular velocity is measured (the image velocity in the image plane is proportional to the angular velocity of the line of sight).
In U.S. Pat. No. 3,511,150 to Whitney et. al., two-dimensional translating of line patterns creates a frequency shift. A single rotating circular line pattern creates all the necessary translating line patterns at specific elongated apertures in a circular mask. The frequency shift is measured on-line using an additional detector measuring a fixed image. The line pattern is divided to two regions, each one adapted for the measurement of different velocity range. The system is basically intended for image motion compensation in order to reduce the image blur in aerial photography. Also, it is useful for missile homing heads.
U.S. Pat. No. 2,772,479 to Doyle describes an image velocimetry system with a frequency offset derived from a grating on a rotating belt.
Laser Doppler Velocimeters generally utilize two laser beams formed by splitting a single source which interfere at a known position. A light-scattering object that passes through the interfering space scatters light from both beams to a detector. The detector signal includes an oscillating element with frequency that depends on the object velocity. The phenomena can be explained in two ways. One explanation is based on an interference pattern that is formed between the two beams. Thus, in that space the intensity changes periodically between bright and dark planes. An object passing through the planes scatters the light in proportion to the light intensity. Therefore, the detected light is modulated with frequency proportional to the object velocity component perpendicular to the interference planes. A second explanation considers that an object passing through the space in which both light beams exist, scatters light from both. Each reflection is shifted in frequency due to the Doppler effect. However, the Doppler shift of the two beams is different because of the different angles of the incident beams. The two reflections interfere on the detector, such that a beat signal is established, with frequency equal to the difference in the Doppler shift. This difference is thus proportional to the object velocity component perpendicular to the interference planes.
It is common to add a frequency offset to one of the beams so that zero object velocity will result in a non-zero frequency measurement. This solves the motion direction ambiguity (caused by the inability to differentiate between positive and negative frequencies) and it greatly increases the dynamic range (sensitivity to low velocities) by producing signals far from the DC components. The frequency offset also has other advantages related to signal identification and lock-on.
U.S. Pat. No. 5,587,785 to Kato, et. al. describes such a system. The frequency offset is implemented by providing a fast linear frequency sweep to the source beam before it is split. The method of splitting is such that a delay exists between the resulting beams. Since the frequency is swept, the delay results in a fixed frequency difference between the beams.
Multiple beams with different frequency offsets can be extracted by further splitting the source with additional delays. Each of these delays is then used for measuring a different velocity dynamic range.
A paper by Matsubara, et al., entitled xe2x80x9cSimultaneous Measurement of the Velocity and the Displacement of the Moving Rough Surface by a Laser Doppler Velocimeterxe2x80x9d (Applied Optics, 36, p. 4516, 1997) presents a mathematical analysis and simulation results of the measurement of the transverse velocity of a rough surface using an LDV. It is suggested that the displacement along the axial axis can be calculated from measurements performed simultaneously by two detectors at different distances from the surface.
In Homodyne/Heterodyne Doppler Measurements, a coherent light source is split into two beams. One beam (a xe2x80x9cprimaryxe2x80x9d beam) illuminates an object whose velocity is to be measured. The other beam (a xe2x80x9creferencexe2x80x9d beam) is reflected from a reference element, usually a mirror, which is part of the measurement system. The light reflected from the object and from the reference element are recombined (usually by the same beam splitter) and directed to a light-sensitive detector.
The frequency of the light reflected from the object is shifted due to the Doppler effect, in proportion to the object velocity component along the bisector between the primary beam and the reflected beam. Thus, if the reflected beam coincides with the primary beam, axial motion is detected.
The detector is sensitive to the light intensity, i.e.xe2x80x94to the square of the electric field. If the electric field received from the reference path on the detector is E0(t)=E0 cos(xcfx890t+xcfx860) and the electric field received from the object on the detector is E1(t)=E1 cos(xcfx891t+xcfx861), then the detector output signal is proportional to (E0+E1)2 =E02 +E0E1+E12.
The first term on the right side of the equation is averaged by the detector time-constant and results in a DC component. The intensity of the reference beam is generally much stronger than that of the light reaching the detector from the object, so the last term can usually be neglected. Developing the middle term:
E0E1=E0E1cos(xcfx890t+xcfx860)cos(xcfx891t+xcfx861) =xc2xdE0E1[cos((xcfx890+xcfx891)t+xcfx860+xcfx861)+cos((xcfx890xe2x88x92xcfx891)t+xcfx860xe2x88x92xcfx861)] 
From this equation it is evident that E0E1 includes two oscillating terms. One of these terms oscillates at about twice the optical frequency, and is averaged to zero by the detector time-constant. The second term oscillates with frequency xcfx890xe2x88x92xcfx891, i.e.xe2x80x94with the same frequency as the frequency shift due to the Doppler effect. Thus, the detector output signal contains an oscillating component with frequency indicative of the measured velocity.
It is common to add a frequency offset to the reference beam. When such a frequency bias is added, it is termed Heterodyne Detection.
U.S. Pat. No. 5,588,437 to Byrne, et al. describes a system in which a laser light source illuminates a biological tissue. Light reflected from the skin surface serves as a reference beam for homodyne detection of light that is reflected from blood flowing beneath the skin. Thus, the skin acts as a diffused beam splitter close to the measured object. An advantage of using the skin as a beam splitter is that the overall movement of the body does not effect the measurement. Only the relative velocity between the blood and the skin is measured.
The arrangement uses two pairs of detectors. Each pair of detectors is coupled to produce a difference signal. This serves to reduce the DC and low-frequency components interfering with the measurement. A beam scanning system enables mapping of the two-dimensional blood flow.
In Optical Coherence Tomography (OCT), a low-coherence light source (xe2x80x9cwhite lightxe2x80x9d) is directed and focused to a volume to be sampled. A portion of the light from the source is diverted to a reference path using a beam-splitter. The reference path optical length is controllable. Light reflected from the source and light from the reference path are recombined using a beam-splitter (conveniently the same one as used to split the source light). A light-sensitive detector measures the intensity of the recombined light. The source coherence length is very short, so only the light reflected from a small volume centered at the same optical distance from the source as that of the reference light coherently interferes with the reference light. Other reflections from the sample volume are not coherent with the reference light. The reference path length is changed in a linear manner (generally periodically, as in sawtooth waveform). This allows for a sampling of the material with depth. In addition a Doppler frequency shift is introduced to the measurement, allowing for a clear detection of the coherently-interfering volume return with a high dynamic range.
In conventional OCT, a depth profile of the reflection magnitude is acquired, giving a contrast image of the sampled volume. In more advanced OCT, frequency shifts, from the nominal Doppler frequency, are detected and are related to the magnitude and direction of relative velocity between the sampled volume (at the coherence range) and the measurement system.
U.S. Pat. No. 5,459,570 to Swanson, et al. describes a basic OCT system and numerous applications of the system.
A paper by Izatt et al., entitled xe2x80x9cIn Vivo Bidirectional Color Doppler Flow Imaging of Picoliter Blood Volumes Using Optical Coherence Tomographyxe2x80x9d (Optics Letters 22, p.1439, 1997) describes an optical-fiber-based OCT with a velocity mapping capability. An optical-fiber beam-splitter is used to separate the light paths before the reflection from the sample in the primary path and from the mirror in the reference path and combine the reflections in the opposite direction.
A paper by Suhara et al., entitled xe2x80x9cMonolithic Integrated-Optic Position/Displacement Sensor Using Waveguide Gratings and QW-DFB Laserxe2x80x9d (IEEE Photon. Technol. Lett. 7 p.1195, 1995) describes a monolithic, fully integrated interferometer, capable of measuring variations in the distance of a reflecting mirror from the measuring device. The device uses a reflecting diffraction element (focusing distributed Bragg reflector) in the light path from the source as a combined beam-splitter and local oscillator reflector. Direction detection is achieved by an arrangement that introduces a static phase shift between signals of the detectors.
Each of the above referenced patents, patent publications and references is incorporated herein by reference.
The present invention in its broadest form provides an Optical Translation Measurement (OTM) method and device, capable of providing information indicative of the amount and optionally the direction of relative translation between the device and an adjacent object. Preferably, the object is at least partly rough and is closely spaced from the device. As used herein, the terms xe2x80x9croughxe2x80x9d or xe2x80x9cdiffusexe2x80x9d mean optically irregular or non-uniform. In particular, the object may have a diffuse opaque or semi-transparent surface such as a paper. This specification deals mainly with determining the translation or velocity of such diffuse surfaces. However, it should be understood that many of the methods of the invention may also be applicable to determination of translation of other types of objects such as small scattering particles, possibly suspended in fluid. Translation of the object means that its rotation in space may be neglected.
In a first aspect of some preferred embodiments thereof, the invention provides heterodyne or homodyne detection of non-Doppler, non-speckle-image signals derived from changes in the phase and/or the amplitude of reflection from an optically irregular surface.
In a second aspect of some preferred embodiments of the invention, applicable to various methods of motion or velocity detection, a system is provided in which a reflector which reflects part of the incident light is placed next to the surface whose motion is to be measured. The reflector provides a local oscillator signal which is inherently coherent with the light which is reflected from the surface. This aspect of the invention is applicable to both Doppler and non-Doppler methods of motion detection.
In a preferred embodiment of the invention, the partial reflector is an apertured reflector in which the illumination of the surface whose motion is measured pass through the aperture. In a preferred embodiment of the invention, the partial reflector covers a portion of the measured surface and has a substantial amount of transmission. In this preferred embodiment of the invention, the reflections from the surface pass through the partial reflector. A combination of apertureing and partial transmission is often useful, especially in preferred embodiments of the invention which utilize the third aspect of the invention.
In a third aspect of some preferred embodiments of the invention, a non-symmetrical transmission pattern is provided to aid in determining the direction of motion of the surface.
In a fourth aspect of some preferred embodiments of the invention, a phase shift is introduced between the reflection from the partial reflector and the reflection from the surface. This phase shift enables the determination of the direction of motion, increases the dynamic range and improves the signal-to-noise ratio.
This phase shift may, in some preferred embodiments of the invention, be dynamic, i.e., time varying. Such phase variations are conveniently performed by moving the reflector either perpendicularly to the surface or parallel to the surface or a combination of both. Also, the movement may be of a pattern on the reflector, e.g.xe2x80x94the movement of a standing wave acting as a grating in a Surface Acoustic Wave (SAW) component. In this respect it is the pattern on the reflector that moves, and not the whole reflector. Alternatively, the phase shift is introduced by periodically varying the optical path length between the reflector and the surface, e.g. by inserting a piezo-electric material in the optical path.
The phase shift may also be a static phase shift. Conveniently, this static phase shift is accompanied by a change in polarization of one of the beams (or a part of the energy in the beam). The direction of motion is determined by a measurement of the phase change and more particularly by measurement of the sign of the phase change. In a preferred embodiment of the invention, the phase of a portion of a beam whose phase is not changed is compared with the phase of a portion which is changed, to determine the direction of motion.
A fifth aspect of some preferred embodiments of the invention provides for Doppler based detection of motion of a surface in a direction parallel to the surface. In this aspect of the invention, a single beam may be incident at an angle to the surface or even perpendicular to the surface.
A sixth aspect of some preferred embodiments of the invention provides for simultaneous two or three dimensional translation detection using a single illuminating beam and a single reflector to provide local oscillator reference beams. In a preferred embodiment of the invention, the signal generated by a single detector is used to determine the translation in two dimensions.
In a seventh aspect of some preferred embodiments of the invention, only a single spatial frequency of the light reflected from the surface is utilized in measurement. Preferably, a spatial filter is provided such that the illumination is reflected from the surface such that substantially only a single spatial frequency of the reflected radiation is detected by the detector.
In some preferred embodiments of the invention which incorporate this aspect of the invention, the spatial filter comprises a lens having a focal point and a pinhole which is placed at the focal point of the lens.
Preferably, the illumination of the surface is collimated and the spatial filter filters the reflected illumination such that only radiation reflected from the surface substantially in a single direction is detected by the detector.
A device, according to a preferred embodiment of the invention, includes a light source, a grating, a spatial filter, a photo-detector, and signal processing electronics. The light source provides at least partially coherent radiation, which is directed toward the surface. An optical grating is placed between the surface and the light source, preferably close to the surface. The light reflected from the surface interferes with the light that is reflected from the grating itself. The detector signal includes an oscillating component, that is representative of the surface translation relative to the optical device. The interference may take place with the normal reflection from the grating or with light diffracted at any of the grating orders. Most preferably, the light passes through a spatial filter prior to detection by the detector. Two dimensional translation measurement may be achieved by using orthogonal reflection orders from a two-dimensional grating or by utilizing two separate gratings for the two directions. A third dimension may be deduced by vector calculation of the translations measured in two different orders at the same axis (e.g.xe2x80x940 and 1, 1 and 2, xe2x88x921 and 1, etc.), or without an additional detector, simultaneously using different signal analysis techniques on the same signal.
Optional detection of the direction of translation (as opposed to it""s absolute magnitude) is preferably achieved by modulating the grating position to provide a frequency offset. Alternatively, a varying optical path length between the grating and the surface introduces the frequency offset. Alternatively, an asymmetric pattern for the grating transmission and appropriate signal manipulation/processing may be used. Alternatively, the direction may be determined by other means.
The method and device of the invention are applicable to a wide range of applications that require measurement of translation. One such application is a xe2x80x9cpadless optical mousexe2x80x9d, that can effectively control a cursor movement by moving the mouse across an optically diffuse surface such as a paper or a desk-top. Another exemplary application for the invention is for a xe2x80x9ctouch-pointxe2x80x9d, that translates finger movement over a device aperture to control a cursor or any other translation or velocity controlled entity.
In accordance with a preferred embodiment of the invention, the measurement apparatus comprises a light source for providing at least partially coherent radiation. The source radiation is directed toward an optical one-dimensional or two-dimensional grating, which is preferably close to the reference surface. The light reflections from the grating and from the surface interfere, and the light is collected through a spatial filter (for example, a lens and a pin-hole at its focal point) into a light-detector. The resulting interference signal contains beats related to the relative translation of the optical apparatus and the surface. Counting the xe2x80x9czero crossingsxe2x80x9d of the oscillating detector signal performs direct measurement of the amount of translation. In preferred embodiments of the invention, the translation is measured directly by counting zero crossings and is thus not subject to errors caused by velocity changes. For preferred embodiments of the invention, substantially instantaneous position determination is established.
In many applications the translation direction as well as its magnitude is required. In a preferred embodiment of the invention this is accomplished by incorporating a phase shifting device (such as a piezoelectric transducer) which creates an asymmetric phase shift pattern (typically a saw-tooth waveform) between the light reflected from the grating and from the surface, enabling simple extraction of the direction information. Alternatively, direction detection is accomplished by using a preferably specially-designed asymmetric transmission pattern for the grating/matrix (such as a saw-tooth transmission or other form as described herein) with appropriate signal processing/manipulation on the detector output signal. An asymmetric transmission pattern provides means for motion direction detection in other velocimetry methods as well, such as speckle velocimetry.
A speckle-free, coherent detection of translation may be determined by collecting the scattered light (the light which passes through the grating and is reflected from the moving surface) with a spatial filter, such as a combination of a focusing lens and a pinhole aperture (or single mode optical fiber) at the focal position of the lens. The light reflected from the surface is combined with a local oscillator light field (which is preferably the light reflected or diffracted by the grating itself), which field is preferably a part of the light beam that also passes through the spatial filter. The interference with the strong local oscillator light source provides amplification of the detected signal by an intensity-sensitive photodetector. This coherent detection method is termed homodyne detection.
The spatial filter is operative to spatially integrate light reflected from the surface to a detector, such that the relative phases of the reflections from different locations on the surface are essentially unchanged when the surface moves with respect to the detector. Furthermore, the phase of a scatterer on the surface (as measured at the detector) depends linearly on the surface translation. Also, the spatial filter is ideally used to filter the local oscillator such that the detector will integrate over no more than a single interference fringe resulting from the interference between the local oscillator and the light reflected from the surface.
In one extreme case, the light incident on the surface is perfectly collimated (i.e.xe2x80x94it is a plane wave). Thus, the spatial filter may simply be a lens with a pinhole positioned at it""s focal point. Any translation of the surface does not change the relative phases of the light integrated by the spatial filter. The local oscillator formed by the reflection or the diffraction from the reflector or grating is also perfectly collimated, so that it can also be passed through the spatial filter (the spatial filter is positioned such that the image of the source falls on or within the pin-hole). This forces a single interference fringe on the detector. No limitations are imposed (with regard to spatial filtering) on the spacing between the reflector and the surface.
In another extreme case, the spacing between the surface and the reflector is negligible. This allows for the use of a substantially non-collimated incident beam while still maintaining the relative phases of the reflections from the surface irrespective of it""s translation and also maintaining the same focusing point for the local oscillator and the reflection from the surface. Optionally, the spatial filter may be implemented with a lens and a pinhole positioned at the image plane of the reflection of the source as a local oscillator.
In order to have (at most) a single speckle integrated by the detector, the pinhole size should not exceed the size of about a single speckle formed by the reflection from the surface (for this reason, the measurement may be termed xe2x80x9cspeckle-freexe2x80x9d). Thus, if the detector itself is small enough, it may serve as an integral part of the spatial filter and a pin-hole is not required.
The requirements of unchanged relative phases and single interference fringe with the local oscillator at the detector can be fulfilled in a multitude of optically equivalent ways. In particular, the requirement may be established using a single converging lens positioned before or after the reflection of the light from the reflector. Alternatively, the lens and the reflector can be combined in a single optical device. Also, a collimating lens may be positioned between the beamsplitter and the surface (i.e.xe2x80x94only light to and from the surface pass through this lens).
Non-ideal spatial filtering (as when the pin hole is too large, or when it is out of focus for either the reflection from the surface or the local oscillator or both), results in deterioration of the signal and possibly the addition of noise to the measurement. The level of deterioration depends on the amount and kind of deviation from the ideal.
In a preferred method according to the present invention, both the surface illumination and the reference light are provided using a single optical element, preferably a grating. The surface and reference light share a single optical path through all of the optical elements in the device. Moreover, the spatial amplitude and/or phase modulation, imposed on the light reaching the surface by the grating, provide additional means for measuring the surface""s translation. In particular, tangential translation can be measured even for specular reflection from the grating, where no Doppler shift exists, and identification of the direction of motion can also be achieved.
There is thus provided, in accordance with a preferred embodiment of the invention, a method for determining the relative motion of a surface with respect to a measurement device, comprising:
illuminating the surface with incident illumination having a coherence length such that the illumination is reflected from portions of the surface, wherein at least part of at least one of the incident and reflected illumination passes through a partially transmitting object that is part of the measuring device;
detecting the illumination reflected from the surface, to generate a detected signal; and
determining the relative motion of the surface parallel to the surface, from the detected signal,
wherein the object and the surface are situated within a distance that is less than the coherence length of the detected illumination;
Preferably, the transmission of the object is spatially varying, more preferably, periodic spatially varying.
Preferably, the object is partially reflecting, part of the incident illumination is reflected or diffracted by the object, as a reference illumination and detection of the illumination is coherent, utilizing said reference illumination.
There is further provided, in accordance with a preferred embodiment of the invention, a method for determining the relative motion of a surface with respect to a measurement device comprising:
placing a partially reflecting object, which is part of the measuring device, adjacent to the surface;
illuminating the object with incident illumination such that part of the incident illumination is reflected or diffracted by the object, as a reference illumination and part is reflected from the surface;
coherently detecting the illumination reflected from the surface utilizing the reference illumination, to generate a detected signal; and
determining the relative motion of the surface parallel to the surface, from the detected signal.
Preferably, the object is a partially transmitting object and at least part of at least one of the incident and reflected illumination passes through the object.
Preferably, the reflection of the object is spatially varying, preferably periodically spatially varying.
In a preferred embodiment of the invention, the object is a grating. Preferably, the grating is placed sufficiently close to the surface such that the surface is in the near field of the grating. Alternatively, the grating is placed sufficiently far from the surface such that the surface is outside the near field of the grating.
In a preferred embodiment of the invention, the detected illumination is at least partly coherent.
There is further provided, in accordance with a preferred embodiment of the invention, a method for determining the relative motion of a surface with respect to a measurement device comprising:
placing an object comprising a grating, which is part of the measuring device, adjacent to the surface;
illuminating the grating with incident illumination such that at least part of the illumination is incident on and reflected from the surface, wherein at least one of the incident and reflected illumination passes through the grating;
detecting the illumination reflected from the surface;
generating a signal in response to the reflected illumination; and
determining the relative motion of the surface parallel to the surface, from the detected signal,
wherein the surface is in the near field of the grating.
In a preferred embodiment of the invention, the illumination reflected from the surface is frequency shifted from that of the illumination reflected from or diffracted by the object and wherein determining the motion comprises determining the motion based on the frequency shift.
In a preferred embodiment of the invention, determining the motion comprises determining variations in the amplitude of the signal with position. Preferably, the motion is determined from zero crossings of the detected signal.
In a preferred embodiment of the invention, the object has a transmission characteristic that is spatially non-symmetric and the method includes determining the direction of the relative motion based on the detected signal.
In a preferred embodiment of the invention, the method includes determining the magnitude and direction of the translation utilizing two detectors which produce different detected signals depending on the direction of the translation. Preferably, the method includes determining the direction of translation from the sign of a phase difference between the different detected signals.
In a preferred embodiment of the invention, the illumination is perpendicularly incident on the surface.
In a preferred embodiment of the invention, the surface is an optically diffusely reflecting surface.
In a preferred embodiment of the invention, the surface has no markings indicating position.
In a preferred embodiment of the invention, the illumination comprises visible illumination. Alternatively or additionally, the illumination comprises infra-red illumination.
Preferably, the method includes determining motion in two dimensions transverse to the surface. Preferably, the method includes determining motion in a direction perpendicular to the surface.
There is further provided, in accordance with a preferred embodiment of the invention, a scanner for reading a document by movement of the scanner over the document comprising:
an optical reading head which detects patterns on the surface of the document; and
an optical detector which determines the motion of the scanner as it is translated across the surface of the document, wherein the optical detector utilizes the method of the invention to determine the translation.
There is further provided, in accordance with a preferred embodiment of the invention, an optical mouse comprising:
a housing having an aperture facing a surface; and
an optical motion detector which views the surface through the aperture, wherein the optical motion detector utilizes the method of the invention to determine the translation of the housing with respect to the surface.